Masseuse generally are highly commissioned workers and they take a share of, i guess, 30~50% of the intake, depending on whether they are good/popular/or not. So from numbers, say you did a 200$ and the masseuse gets 40%, that's 80$. Then a tip of another 10-20% will bump up his/her hour pay by 25~50%..
I think really depends on whether its a regular one or you just walk in.. for regular masseuse i guess regular tipping will help you in getting some preference considerations in scheduling during the busier weekends.. But its not a must in HK.